Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 8 Oct 2018 10:42:54 -0700 | Subject | Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function" |
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On Mon, Oct 8, 2018 at 10:30 AM Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote: > > On 8 October 2018 at 19:25, Andy Lutomirski <luto@amacapital.net> wrote: > > On Mon, Oct 8, 2018 at 9:40 AM Peter Zijlstra <peterz@infradead.org> wrote: > >> > >> On Mon, Oct 08, 2018 at 09:29:56AM -0700, Andy Lutomirski wrote: > >> > > >> > > >> > > On Oct 8, 2018, at 8:57 AM, Peter Zijlstra <peterz@infradead.org> wrote: > >> > > > >> > > On Mon, Oct 08, 2018 at 01:33:14AM -0700, Andy Lutomirski wrote: > >> > >>> Can't we hijack the relocation records for these functions before they > >> > >>> get thrown out in the (final) link pass or something? > >> > >> > >> > >> I could be talking out my arse here, but I thought we could do this, > >> > >> too, then changed my mind. The relocation records give us the > >> > >> location of the call or jump operand, but they don’t give the address > >> > >> of the beginning of the instruction. > >> > > > >> > > But that's like 1 byte before the operand, right? We could even double check > >> > > this by reading back that byte and ensuring it is in fact 0xE8 (CALL). > >> > > > >> > > AFAICT there is only the _1_ CALL encoding, and that is the 5 byte: E8 <PLT32>, > >> > > so if we have the PLT32 location, we also have the instruction location. Or am > >> > > I missing something? > >> > > >> > There’s also JMP and Jcc, any of which can be used for rail calls, but > >> > those are also one byte. I suppose GCC is unlikely to emit a prefixed > >> > form of any of these. So maybe we really can assume they’re all one > >> > byte. > >> > >> Oh, I had not considered tail calls.. > >> > >> > But there is a nasty potential special case: anything that takes the > >> > function’s address. This includes jump tables, computed gotos, and > >> > plain old function pointers. And I suspect that any of these could > >> > have one of the rather large number of CALL/JMP/Jcc bytes before the > >> > relocation by coincidence. > >> > >> We can have objtool verify the CALL/JMP/Jcc only condition. So if > >> someone tries to take the address of a patchable function, it will error > >> out. > > > > I think we should just ignore the sites that take the address and > > maybe issue a warning. After all, GCC can create them all by itself. > > We'll always have a plain wrapper function, and I think we should just > > not patch code that takes its address. So we do, roughly: > > > > void default_foo(void); > > > > GLOBAL(foo) > > jmp *current_foo(%rip) > > ENDPROC(foo) > > > > And code that does: > > > > foo(); > > > > as a call, a tail call, a conditional tail call, etc, gets discovered > > by objtool + relocation processing or whatever and gets patched. (And > > foo() itself gets patched, too, as a special case. But we patch foo > > itself at some point during boot to turn it into a direct JMP. Doing > > it this way means that the whole mechanism works from very early > > boot.) > > Does that mean that architectures could opt out of doing the whole > objtool + relocation processing thing, and instead take the hit of > going through the trampoline for all calls? >
I don't see why not.
--Andy
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